The present invention relates to polypropylene films and biaxially oriented films with improved properties. Polypropylene produces films that have numerous uses including, for example snack food packaging, cigarette overwrap, electronic components wrapping, packaging tape, and shrink film. The polymers normally employed in the preparation of biaxially oriented films are isotactic homopolymers with high stereoregularity, although on some occasions the use of syndiotactic polymers has been proposed. Also suitable are co-polymers of isotactic polypropylenes with a small content of ethylene (mini-random co-polymers).
Isotactic polypropylene is one of a number of crystalline polymers that can be characterized in terms of the stereoregularity of the polymer chain. Various stereo specific structural relationships denominated primarily in terms of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers of various monomers.
Isotactic polypropylene is conventionally used in the production of relatively thin films in which the polypropylene is heated and then extruded through dies and subject to biaxial orientation by stressing the film in both a longitudinal direction (referred to as the machine direction) and lateral direction sometimes referred to as the tenter direction.
The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is very low in crystallinity and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
The isotactic polymers normally employed are typically prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al.
U.S. Pat. No. 5,573,723 to Peiffer discloses a process for producing biaxially oriented polypropylene film based on an isotactic polypropylene homopolymer or propylene ethylene co-polymers.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Traditional supported catalysts are the so-called xe2x80x9cconventionalxe2x80x9d Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as disclosed, for example, in the aforementioned patents to Myer et al.
Alternative types of catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a xe2x80x9cconventionalxe2x80x9d supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamsoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. No. 5,308,811 to Suga et al and U.S. Pat. No. 5,444,134 to Matsumoto.
Polypropylene films can be produced by methods known in the art and include biaxially oriented films as well as blown films. The biaxially oriented films are preferably produced in tenter frame processing lines or double bubble processing lines.
Polypropylene compositions which can be oriented at variable process conditions, particularly over a range of machine direction orientation draw ratios and transverse direction orientation oven temperatures, are desirable for a number of reasons. Film manufacturers have the flexibility to vary one or more processing conditions within an acceptable range for a particular film production run. In addition, the possibility of a web break during the orientation process is lessened, resulting in lower processing line start up costs. For example, a standard machine direction orientation draw ratio may be about 5 times the original length of the polyolefin film. A standard transverse direction oven temperature is about 166xc2x0 C. These processing conditions are considered standard in that the occurrence of web breaks in the film are infrequent.
The ability to orient film compositions under variable conditions, for example a machine direction draw ratio within the range of its standard up to 9 times the original length, and a transverse direction oven temperature within the range of its standard to about 20xc2x0 C. less than the standard, gives manufacturers greater flexibility. A further processing advantage is the ability to draw the film at higher line speeds thereby decreasing the manufacturing time.
To achieve processabillity at high line speeds metallocene polymers can be blended with the polypropylene in an effort to maintain film properties and gain a wider process window. However, this technique can be a costly means to accomplish both goals.
Biaxially oriented films can have a number of properties to their advantage during and after the machine processing steps. A relatively low coefficient friction is desirable, both during the biaxially orientation procedure and in the use of the ultimately produced biaxially oriented film for end use applications. A relatively high stiffness, as indicated by the tensile modulus in both the machine direction and the transverse direction is usually advantageous. Relatively low permeabilities to gas and water are desirable. In addition, a high shrinkage factor of the processed film, while undesirable in some cases, can be advantageous in others, such as where the film is used in stretch wrapping of food products, electrical components, and the like.
Properties of the resulting film product can be dependent to a certain degree on the particular process conditions under which the polypropylene composition was manufactured. For example, a stiffer film with a higher shrinkage factor and better barrier properties would result from an orientation process incorporating a larger machine direction orientation draw ratio. Likewise, the transverse direction orientation oven temperature would affect the properties of the resulting oriented film product, particularly improving the shrinkage factor.
The physical and optical properties of films are important in the film industry and should fall within certain parameters for different film applications. The optical properties include haze, clarity, and gloss. Haze is a phenomenon of light scattering and arises from local variations in the refractive index. Haze is defined as the relative fraction of scattered intensity from the dispersed particles in all directions, being detected in a range of wide angle, to the incident light intensity. Clarity is a measure of contact clearness or see-through quality and is different from haze due to the direct transmittance of light. For example, some films may indeed be hazy but appear clear as the film is in contact with the contents of a package. Unlike haze, clarity is distance dependent so that the thinner the film, the better the contact clarity. Gloss is defined as the ratio of the reflected light intensity from the film at a specific angle of incidence light to that of a standard with the ideal smooth surface.
Nucleating agents are used to increase the stiffness of processed materials. Nucleating agents may also improve the optical and barrier properties of the resulting materials. Various nucleating agents have been used with polypropylene materials. For example, U.S. Pat. Nos. 5,300,549 and 5,319,012 to Ward et al. (the Ward patents) disclose the use of dicarboxylic and monocarboxylic acids for the subsequent manufacture of shaped articles. U.S. Pat. No. 5,856,386 to Sakai et al. uses rosin acid metallic salts as the nucleating system. The above identified patents are incorporated herein by reference.
Most nucleating agents (sodium benzoate, talc etc.) are particulate in nature, and more specifically, are ground to the appropriate particle size to be used in polyolefins as a nucleating agent. They may have a particle size distribution consisting of a mean size of 2 micron with a top size of 10 micron, for example. Although this is not a problem in converting systems such as molding or thick extrusion (sheet or Pipe), this can be a problem in thin gage film. Most particulate nucleants in film cause the film to break during stretching.
It is difficult to disperse particulate nucleating agents and get effective homogeneous nucleation, even at very low levels. Also, the crystallization characteristics of the nucleated film, in most cases, are so rapid that the film tends to break during orientation of the film, due to the narrow stretch window caused by both the nonuniform dispersion of the agent, and the high rate of crystallization.
To solve this problem, non particulate systems that are compatible with the dynamics of film stretching (slower nucleation rates) need to be used. The nucleating systems disclosed in the Ward patents exemplify some of the non-particulate nucleating systems commercially available.
Nucleating agents can be added directly to molten polypropylene, for example, in a xe2x80x9cmelt blendxe2x80x9d or by other methods known in the art.
The above nucleating systems and methods disclose different concentrations of nucleation agents depending upon the agents selected. The carboxylic acid agents of the Ward patents use exemplified concentrations of 2500-5000 ppm. Rosin acid metallic salts of the Sakai patent were exemplified in concentrations of 3000-9000 ppm.
Surprisingly, it has been discovered that improved polypropylene films and biaxially oriented polypropylene (BOPP) films may be formed using non-particulate nucleating systems in low concentrations with mini-random polypropylene.
It is therefore the object of this invention to provide compositions useful for production of polypropylene films and preferably BOPP film of improved tensile strength comprising mini-random polypropylene and low concentrations of the nucleating agents.
A method of making a mini-random polypropylene product for use in making films and preferably biaxially oriented films and more preferably biaxially oriented films wherein a tenter-frame processing line is employed. The method includes the addition of a nucleating system in a mini-random polypropylene that allows for better processability and improved tensile properties. In one embodiment the nucleating system comprises a blend of a monocarboxylic acid nucleating agent and dicarboxylic acid nucleating agent. The nucleating system is combined with the polypropylene material and the resultant hot melt blend is further processed into biaxially oriented film. Alternately, the hot melt blend is formed into a convenient raw material for later use in a biaxial oriented film production facility. The resulting films possess superior processing and material characteristics.